This invention relates to the field of electro-optical detecting apparatus which utilizes "mosaic" detector arrays to provide surveillance of an extensive scene. The mosaic detector arrays are large numbers of closely spaced individual photodetector elements arranged in essentially a two-dimensional, or planar, array.
In patent application Ser. No. 187,787, filed Sept. 16, 1980 and assigned to the assignee of this application, a detector array module is disclosed which comprises a stack of semiconductor chips, or layers, extending at right angles to the focal plane of the detector array, which array comprises a multiplicity of separate photo-detectors each in electrical contact with a single electrical lead formed as one of a multiplicity of such leads which appear on the focal plane ends of the stacked chips. Each of the chips as integrated electronic circuitry which processes signals from the detectors prior to transmitting output signals to electronic processing circuits located remote from the focal plane assembly. The complete focal plane assembly is constructed by combining a large number of the modules.
The goal of such structures is to derive individual detector signals from photo-detectors which have a very small center-to-center distance, in order to provide optimum resolution of the incoming optical information. The desired spacing of the detectors is determined by the size of the optics system in the sensor. Longer focal lengths in the optics system permit greater spacing of the detectors. There is a design trade-off between the dimensions of the focal plane and the optics, on the one hand, and the size and spacing of the detectors, on the other hand.
Compact optical receiving systems require small, closely spaced detectors. Except for the diffraction limit, the desire is to increase to the maximum the number of individual detectors in a given focal plane area. It is desirable to obtain the smallest possible F-number, which is the ratio of focal length of the optics to the diameter of the input aperture. The optimum relationship is as follows: d=0.1.multidot..lambda..multidot.F/no., where d=detector size in mils, and .lambda.=wavelength in microns. For infra-red radiation in the 8-12 micron region, with an F/no. of 1.5, the ideal center-to-center detector distance is about 1.5 mils; and for infra-red radiation in the 3-5 micron range, with an F/no. of 1.5, the ideal center-to-center detector distance is about 0.75 mil. In units presently under development, the minimum distance is about 4 mils.
The primary deterrent to reducing the distance is the required thickness of each layer. If the layers are too thin, they will be very fragile, and will therefore be extremely difficult to handle during fabrication of the multi-layer modules. The problem of layer fragility is, in fact, a significant one where a 4 mil dimension is used.
The purpose of the present invention is to significantly improve the strength of the layers and/or the closeness of the detector centers. In other words, the goal is to make it possible to bring the detectors closer together, given the limitation imposed by the strength requirement of the layers.